Handheld pipettes are commonly used to dispense or transfer small but accurately measured quantities of liquids.
U.S. Pat. No. 5,700,959, for example, describes a commercially available single channel air displacement manual pipette. Such pipettes generally include an elongated hand-holdable pipette body housing an upwardly spring biased plunger unit. The plunger unit is supported for axial movement in the pipette body between a first or upper stop position in which an end portion of the plunger unit extends from an upper end of the pipette body. A pipette user grips the pipette body with his or her thumb over the exposed end of the plunger unit. Downward thumb action on the plunger unit moves the plunger unit downward from its upper stop position against the upward bias of a return spring toward a home position, and on against the return spring and a second spring to a second or a lower stop position at which the measured fluid is expelled from a disposable tip secured to the pipette.
In the commercially available pipettes, as described in the foregoing patent, the home position is defined by a “soft” stop. The soft stop comprises a second relatively stiff spring mechanism, often referred to as a “blowout” spring, within the pipette body which is installed in a somewhat preloaded state, but further activated when the plunger unit reaches the home position. As the pipette user manually moves the plunger unit from its upper stop position by pressing downwardly with his or her thumb on the exposed end of the plunger unit, the pipette user can “feel” an increased resistance to movement of the plunger unit associated with an activation of the second spring assembly opposing further downward movement of the plunger unit. The position of the plunger unit where the user feels the activation of the second spring mechanism defines the home position for the plunger unit. Continued movement of the plunger unit beyond the home position to the lower stop position is resisted by a combination of the return spring and the second spring mechanism. The volume of the pipette is defined by the distance between the upper stop and the soft “home position” stop, and accordingly, the tactile feel of the home position—the transition between the two spring resistances—is an important characteristic of a manual pipette.
Air displacement pipettes are the most common variety of handheld pipettes. In an air displacement pipette, a controllable piston is mounted for movement axially within a chamber in the pipette; the piston moves in response to either manual control (as described above) or motorized electronic control. Typically, the piston moves in a chamber in the liquid end, or shaft, of the pipette, to which disposable pipette tips may be mounted.
An air tight seal is formed between the piston and the shaft. With such a seal in place, axial movement of the piston will vary the size of the airspace within the shaft. Moving the piston downward, into the shaft, will reduce the airspace and force air out of the shaft through an open distal end. Moving the piston upward, out of the shaft, will increase the airspace and cause air to be drawn into the shaft through the open end. The seal between the piston and the shaft is generally formed with a compressed O-ring, a skirted seal, a lip seal, or a similar structure, fabricated from a material that provides satisfactory long-term performance. For example, a piston seal structure may be made from polyethylene combined with PTFE, which has been found to offer good sealing performance and wear resistance and reliability over a period of months to years. Other configurations are possible, including various dry or lubricated seals.
A disposable pipette tip is then sealed to a nozzle at the open distal end of the shaft. Then, as the piston is moved within the shaft, air—or a measured quantity of liquid equal in volume to the displaced air—is drawn into or forced out of the tip. With both the piston and the tip sealed to the shaft, the only entry and exit path should be the distal open end of the disposable pipette tip. Because of the sealed system, air displacement pipette may be used to make accurate and precise measurements, and to move carefully calibrated quantities of liquids.
In pipetting liquids with traditional manual air displacement pipettes, the pipette user grasps the pipette housing with his or her thumb on top of the exposed end of the plunger unit. Exerting downward thumb pressure on the plunger unit, the user moves the plunger unit away from the upper stop position against the force of the return spring. The user detects the home position for the plunger unit during movement of the plunger unit away from the first stop position by sensing the start of an increase in the downward force required to move the plunger unit. Such increase force is the result of movement of the plunger unit against the return spring and the preloaded second spring mechanism, commonly referred to as a “blowout” spring mechanism. Then, with the tip inserted in the liquid, the user manually controls the rate of return of the plunger unit from the home position to the upper stop position.
Subsequently, to dispense the liquid, the user removes the tip from the liquid and maneuvers it to a position above a receptacle, then depresses the plunger unit gradually to the soft stop at the home position, then beyond the home position through a blowout stroke. The volume of liquid discharged during the downward main stroke between the upper stop position and the home position should, in theory, be equal to the volume of liquid aspirated while moving the plunger unit upward over the same stroke. In practice, however, some liquid may cling to the disposable tip, either on an interior surface or as a droplet on the bottom, or both. Additional air discharged from the pipette during the blowout stroke, between the home position and a fixed lower stop, assists in removing this remaining liquid. However, in most commercially available pipettes, the blowout stroke is relatively short—as a practical consequence of the limited possible stroke length when the plunger unit is to be controlled by a user's thumb. Such a short blowout stroke may not be sufficient to remove substantially all of the remaining liquid. Any remaining liquid that has not been successfully dispensed may tend to adversely affect the accuracy of a liquid dispensing operation performed via pipette. This is particularly true in the case of low-volume pipettes, especially those handling 50 μl or less. With low-volume pipettes, the ratio of adhering liquid to the desired sample size may be especially high.
To remove the remaining liquid—to the extent it is hanging as a drop at the bottom of a tip—a user may attempt to “touch off” and tap the distal end of the tip against the side of the receptacle. However, it may not always be practical to touch off in all circumstances, and not all adhering liquid may be removed this way. Automated or robotic liquid handling systems may not have the freedom to touch off against the side of a receptacle, or a protocol may not permit it. Moreover, liquid transferred to the side wall of a receptacle in this way might remain as a separate drop on the side wall, and in some cases might not rejoin the rest of the discharged sample as a user might desire.
This problem is well known and there have been some attempts made to solve it. U.S. Pat. No. 5,696,330 to Heinonen discloses a manual air displacement pipette that includes two concentric pistons—a “dosing piston” 18 that performs the primary liquid aspiration and dispending between the piston's upper position and its home position, and a secondary and separately movable “removing piston” 13 that moves during the blowout stroke to expel additional air and detach droplets. During a downward stroke of the Heinonen pipette, only the dosing piston is operative between the upper stop and the home position. At the home position, the dosing piston engages and causes movement of the secondary removing piston. Although this design will certainly discharge more air during blowout, it includes an excess of moving parts with tight tolerances, which may lead to long-term unreliability concerns and additional manufacturing expenses.
U.S. Pat. No. 8,318,108 to Suovaniemi et al. attempts to solve the problem in a slightly different manner—by using a single piston, but accelerating it during a blowout stroke. This too will discharge more air, more quickly during blowout, which will indeed tend to provide more effective blowout characteristics. However, because piston movement in a traditional manual handheld pipette is controlled by the user, the Suovaniemi technique is best implemented in an electronic pipette under motorized control. It is possible to design a fully manual pipette with this movement characteristic imparted to the piston entirely through mechanical means through a two-speed linkage, but this design would be more complex and once again employ more moving parts. And to discharge more air during a blowout stroke, even if accelerated, it may be necessary to lengthen the piston stroke of the pipette, which may in turn require lengthening the pipette to a size greater than a user might otherwise prefer.
Accordingly, there is a continuing need for a manual air displacement pipette with enhanced and improved blowout characteristics. Such a pipette would offer an increased ability to remove any remaining or adhering liquid from a pipette tip without substantially increased complexity, size, cost, or operational difficulties.